1. Field of the Invention
The present invention relates in general to an apparatus for controlling an automatic transmission of a motor vehicle, and more particularly to techniques for controlling a hydraulically operated frictional coupling device during a shifting action of the automatic transmission.
2. Discussion of the Related Art
There is known an automatic transmission for motor vehicle, which is adapted to effect a shift-up action by releasing a one-way clutch while engaging a certain hydraulically operated frictional coupling device, or by releasing the frictional coupling device while engaging another hydraulically operated frictional coupling device. This type of automatic transmission tends to suffer from a shifting shock upon such a shift-up action during acceleration of the vehicle with an accelerator pedal being depressed. Thus, there has been a need to minimize such a shifting shock of the automatic transmission.
For controlling an automatic transmission with a reduced shifting shock, there is proposed an apparatus for controlling the pressure of a hydraulically operated frictional coupling device which is released to achieve a given shifting action of the automatic transmission. This control apparatus is adapted to first rapidly reduce the pressure of the frictional coupling device to a predetermined level, and then continuously reduce the pressure at a relatively low rate so that the input speed of the automatic transmission coincides with a target value. An example of such a known automatic transmission control apparatus is disclosed in JP-A-63-397849.
In controlling the pressure of the frictional coupling device during a shifting action of the automatic transmission, the known control apparatus does not take into account factors other than the input shaft of the automatic transmission, which factors influence the behavior of the automatic transmission during the shifting action. Such factors include the temperature of the working fluid for operating the frictional coupling device, and the chronological change of the operating or engaging characteristic of the coupling device. The automatic transmission controlled by the known control apparatus tends to suffer from a shifting shock or an overshoot (temporary rise) of the engine speed, particularly when the automatic transmission is shifted up during acceleration of the motor vehicle with the accelerator pedal placed in a depressed state. Thus, the known control apparatus is not satisfactory to assure sufficient stability or smoothness of the shifting action of the automatic transmission.